This invention relates to a process for the removal of unsaturated hydrocarbons from a stream of anhydrous hydrogen halide.
Commercially produced hydrogen halides, such as hydrogen chloride, normally contain small amounts, e.g., up to about 0.5% by volume of organic impurities, such as ethylene, propylene or other low-boiling alkylenes, acetylene and chlorinated hydrocarbons such as chloroethylene and ethyl chloride.
Known methods of removing such organic impurities from hydrogen chloride include fractional liquefaction, distillation, scrubbing and catalytic conversion. These methods however, may not be satisfactory when an ultra-high-purity hydrogen chloride is desired, e.g., a hydrogen chloride containing less than about 0.005% by volume organic impurities.
The decomposition of various organic compounds by subjecting them to laser radiation has long been known in the art. Thus, "The Photo-Induced Pyrolysis of Ethylene in cw CO.sub.2 Laser Beam", Tardieu de Malessye et al, Chemical Physics Letters, Volume 43, No. 3, Sept. 15, 1976, discloses the decomposition of ethylene by subjecting it to laser radiation produced by a CO.sub.2 laser operated at a wavelength of 10.6 micrometers. The ethylene was decomposed to gaseous hydrocarbons and hydrogen. Carbon and tars were also produced.
"High Temperature Pyrolysis By Laser Gas Break-down", Kojima, Ind. Eng. Chem. Prod. Res. Dev., 1981, 20, pages 396 to 399 discloses the decomposition of C.sub.1 -C.sub.6 hydrocarbons by laser radiation form a CO.sub.2 laser. The radiation induces plasma formation, resulting in decomposition of the target gas. The article discloses the application of laser radiation to various hydrocarbons, some of which do not absorb radiation of the wavelength produced by a CO.sub.2 laser. When a molecule is irradiated with radiation at a wavelength at which it readily absorbs, decomposition occurs in a unique way. In the experiments reported by the article, ethylene, which absorbs strongly at the wavelength of radiation produced by a CO.sub.2 laser, when irradiated at that wavelength apparently decomposed into carbon and hydrogen.
Chemical Abstracts 86:197828f, discloses the investigation of chemical reactions in C.sub.2 H.sub.4 irradiated with CO.sub.2 laser pulses. A focused laser beam generated a plasma which converted C.sub.2 H.sub.4 to C.sub.2 H.sub.2. If the beam were not focused, there was no dehydrogenation but the C.sub.2 H.sub.2 was capable of reacting with HCl.
Chemical Abstracts 91:115251d, discloses the irradiation of C.sub.2 H.sub.3 Cl by CO.sub.2 laser radiation to yield C.sub.2 H.sub.2 and HCl.
U.S. Pat. No. 3,977,952 discloses the decomposition of carbon-containing compounds by subjecting them to radiation of a wavelength of 20 to 600 nm in the presence of water and oxygen in excess of the stoichiometric amount necessary for complete oxidation. Although a catalytic quantity of HCl may be present, the patent teaches that it is desirable to maintain the concentration of HCl at a predetermined level during the course of the reaction. Examples disclose the concentration of HCl to be very low. Additionally, the HCl is introduced in the form of an aqueous solution rather than in anhydrous form. Moreover, the carbon-containing compounds are converted, e.g., to CO.sub.2, and are not decomposed to atomic constituents.
U.S. 4,063,896 discloses the removal of COCl.sub.2 as an impurity from BCl.sub.3 by subjecting the BCl.sub.3 to laser radiation produced by a CO.sub.2 laser. The BCl.sub.2 resonates with the radiation produced by a CO.sub.2 laser, transferring the absorbed energy to the COCl.sub.2, which results in the dissociation of the COCl.sub.2. The patent also teaches that because C.sub.2 H.sub.4 also resonates with such radiation, it can be used as a diluent to affect energy transfer to the COCl.sub.2.
U.S. Pat. No. 4,230,546 discloses the dissociation of various molecules by laser radiation. The desirability of matching the frequency of the laser to the absorption characteristics of the molecule being irradiated is disclosed.
Although the prior art establishes that ethylene can be decomposed with CO.sub.2 laser radiation, there is lacking a suggestion that small amounts thereof in a gaseous hydrogen halide can be decomposed into products readily removable from the hydrogen halide. In fact, if anything, the opposite is suggested.